The present invention relates in general to the apportionment of nearly identical distribrtions of radiation among a number of receiving optical fibers and, more specifically, to a radiation mixing device that efficiently couples radiation from the source or interferometer of a Fourier Transform Infrared Spectrometer (FT-IR) to a multiplicity of optical fibers so that each fiber receives substantially identical spectral, angular and intensity distributions of radiation.
The recent development of heavy metal fluoride glasses ("HMFG") now makes it possible to obtain spectra from remote sensors, such as transmission cells, for wavelengths from the visible out to more than four micrometers. This technology is not yet widely used, but the potential for large savings in process control applications in the petroleum, chemical and food industries hold promise for the rapid growth of remote spectroscopy.
A typical fiber optic circuit conducts light from the source and wavelength selection portion of the analyzer to the remote sensor and back through another fiber to the detector portion of the analyzer. The relatively high cost of the HMFG fibers makes it advantageous to use single, small diameter fibers to connect the analyzer to the sensor. A small diameter fiber carries less energy than does a large fiber and requires the use of a more sensitive analyzer such as a Fourier Transform Infrared Spectrometer. Since the sensitivity of the analyzer is generally related directly to the cost of the analyzer, it becomes economically desirable to share a single analyzer among several sensors.
Furthermore, there is a need for easily and reliably switching from one fiber circuit to another and for switching between the sample circuit and a reference circuit. This is required in most spectrometers for maintaining the accuracy necessary for quantitative measurements.
There are, however, some considerations that complicate optical switching in an FT-IR spectrometer. The heart of an FT-IR spectrometer is a Michelson interferometer. A typical layout of a traditional FT-IR spectrometer is shown in FIG.
A source 1 radiates light onto a collimator 2 which sends a collimated beam into the Michelson interferometer 3. Optical element 4 focuses the beam onto the sample 5 in the sample chamber 6. From there the beam continues on to optical element 7 which in turn focuses tha beam onto detector 8.
A typical layout of an FT-IR spectrometer configured for remote spectroscopy using a single optlcal fiber is shown in FIG. 2. It differs from the tradLtional FT-IR spectrometer in that optical element 4 is arranged to focus the beam on the end of an optical fiber 9 which carries the light to a remote sample sensor 10 from which light is carried by a second fiber 11 back to optical element 7 which focuses the light onto detector 8. No basic difficulties are encountered by introducing the fibers that cannot be accommodated by appropriate selection of the optical elements and the detector.
The difficulties are greater, however, when one attempts to use more than one fiber in connection with the Michelson interferometer. One approach is to use two optical switches 12 and 13 as depicted in FIG. 3. Three remote fiber optical circuits are shown by way of example. Optical element 4 focuses the beam onto the input of the optical switch 12. Depending on the position of the optical switch, a portion of the beam is directed into one of the fibers 91, 92 or 93, and is carried by the fiber to a corresponding optical circuit, such as a remote sensor indicated as 101, 102, or 103, and then back to optical switch 13 along corresponding fibers 111, 112, or 113. Provided that switch 13 is set for the corresponding fiber, the beam is directed by the switch onto optical element 7 which focuses it onto detector 8. Although this approach is a straightforward extension of the single fiber system depicted in FIG. 2, it suffers from requirlng two optical switches that are usually rather expensive when constructed to be reliable for the analog signals used in fiber spectroscopy.
A second approach is to use a single optical switch and make use of the large etendue or light gathering power of the FT-IR spectrometer. The complicationr introduced by this approach are related to the spatial extents of the source and the bundle of fibers into whicl the light is launched. This approach is depicted in FIG. 4.
The optics in FIG. 4 have been expanded to show in more detail the passage of beams through the irterferometer 3 and the relationship between the field stop (Jacquinot stop) and the pupil stop. The light from the extended source 1 is focussed by element 14 onto field stop 16. Optical element 2 not only collimates the light from each point within the field stop, but also forms images of the pupil stop 15 typically at or near the mirrors 17 and 18. The pupil limits the angular extent of the rays passing through the field stop. Reciprocally, the field stop limits the angular extent of the rays passing through any subsequent image of the pupil. Due to the spatial extents of the source and the field stop, the beam within interferometer is not truly collimated, but diverges by typically one or two degrees.
It should be noted that the interference of beams within the interferomoter depends not only on wavelength (the parameter of interest to the spectroszopist), but also on the positions of the mirrors and the angle of refleotion from the mirrors. An FT-IR speotrometer achieves its highest resolution as the Jaoquinot interferometer is reduced. It should be noted further that since the optical throughput of a typical FT-IR spectrometer is much greater than the throughput of an optical fiber, a multiplicity of fibers can be coupled to the interferometer without degrading its resolution.
A short focal length element 19 is pcsitioned about one focal length ahead of where the field stop image would be formed by optical element 4 and the fiber ends 20 are positioned at the small image of the field stop formed by element 19. Each fiber has a different lccation within the field stop image. Consequently, the rays entering each fiber correspond to rays having a different direction through the interferometer. Moreover, since the field stop is located at an image of the source 1, each fiber samples a different part of the source.
In principle, one can eliminate the problem of different sets of angles in the interferometer by positioning the fibers on a circle for which the center is aligned with rays that are perpendicular to the mirrors 17 and 18 in the interferometer. However, in practice this is a difficult alignment to achieve, and if it is not achieved, the wavelength accuracy from each fiber circuit can be different due to the differences in mean path length resulting from the differences in angles in the interferometer. Furthermore, even if the angular variation is eliminated, the fact that each fiber is sampling radiation from a different part of the source will lead to long-term instability due to changing position and temperature variations of the source.
FIG. 4A shows a variation which in principle eliminates these problems. Optical element 4 focuses the field stop 16 onto the short focal length optical element 19 which focuses a small image of the pupil 15 onto the ends 20 of the tightly clustered fibers 91, 92, and 93. The light hitting each fiber is now averaged over a substantially greater part of the source limited usually by the numerical aperture of the fiber. In principle, the fibers can be aligned so that the axis of each fiber passes through the same point in the image of the field stop to assure that the mean path length of the rays in the interferometer are identical for each fiber. In practice this is difficult because it does not permit the fiber axes to be parallel at the ends 20, and, furthermore, it is complicated by the fact that in many spectrometers the image of the pupil is partially obscured by small mirrors mounted in the beam in the interferometer to monitor the position of the moving mirror.
It is accordingly a general object of the invention to provide a solution to these problems.
It is a specific object of the invention to provide a fiber optical mixer.
It is another object of the invention to provide a fiber optical mixer for use in Fourier Transform Infrared Spectrometers (FT-IR).
It is a further object of the invention to provide a fiber optical mixer that provides substantially identical spectral, angular, and intensity distributions of radiation to a plurality of optical fibers.